A new study challenges the popular idea that dysfunction in dopamine — a chemical that controls the brain’s reward and pleasure centers — is the main cause of attention deficit hyperactivity disorder (ADHD). The U.K. researchers suggest instead that the primary cause of ADHD is found in structural differences in the brain’s grey matter.

Dopamine is a chemical produced in the brain that is necessary for concentration or prolonged attention, working memory and motivation. It helps carry signals between brain cells by attaching to dopamine cell receptors — special entry-points in cell membranes that can only be opened by that particular molecule.

Ritalin, one of the more popular medications approved to treat ADHD, raises levels of dopamine, causing more to bind to the cells and therefore increasing the communication between them.

“These findings question the previously accepted view that major abnormalities in dopamine function are the main cause of ADHD in adult patients.

“While the results show that Ritalin has a ‘therapeutic’ effect to improve performance, it does not appear to be related to fundamental underlying impairments in the dopamine system in ADHD,” said co-author Trevor Robbins, Ph.D., director of the MRC Centre for Behavioural and Clinical Neuroscience Institute.

During the study, researchers used a combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) to measure grey matter and dopamine receptors and determine how the drug methylphenidate (Ritalin) affected dopamine in individuals with ADHD and those without.

Both study groups were given either a dose of Ritalin or a placebo. The study was double-blinded, which means neither the participants nor the clinicians who administered the medication knew whether they were working with Ritalin or the placebo.

Before and after taking their given dose, participants were tested on their ability to concentrate and pay attention over a period of time.

The researchers discovered that both the ADHD patients and the controls who were given Ritalin showed similar increases of dopamine in their brain, as well as similar levels of improvement in attention and concentration.

The findings also showed that although participants with ADHD had significantly less grey matter in the brain, and performed much worse in the attention tests than the healthy controls, they had similar levels of dopamine receptors in an area of the brain called the striatum. Ritalin raised dopamine levels in this area to the same degree.

This important finding suggests there was not necessarily any dysfunction in dopamine.

The researchers found it interesting that Ritalin also increased sustained performance in some of the healthy controls, suggesting that the overall ability of the drug to increase attention in both ADHD and healthy controls was related to the rise in dopamine it caused in the striatum.

Study leader Barbara Sahakian, Ph.D., said the findings are significant because they show Ritalin improves attention and concentration regardless of whether people have ADHD or not.

“These new findings demonstrate that poor performers, including healthy volunteers, were helped by the treatment, and this improvement was related to increases in dopamine in the brain,” she said.

The researchers hope these results will improve our understanding of the cause of ADHD and improve future treatments.

Source: University of Cambridge